All suggestions for changing mons, sets, etc. are appreciated. Now, onto the team.
Ability: Infiltrator
Tera Type: Ghost
EVs 4 Atk / 252 SpA / 16 Spe
Hasty Nature
- Dragon Darts
- Shadow Ball
- Flamethrower
- U-turn
Dragapult works as the offensive pivot of the team. I chose Life Orb as the item because it provides extra power, allowing it to deal big damage. I used it over Specs/Band because I like the flexibility Life Orb provides on an offensive pivot. Dragon Darts is a good dragon move that can threaten dragons and nwutral mons, when weakened enough. Shadow Ball is Dragapult's most spammable move, and Ghost Tera gives it even more power. Flamethrower gives good coverage against Grass Types, which Rotom-W struggles with. U-turn generates momentum for the team and creates a VoltTurn core with Rotom-W.
Rotom-Wash @ Leftovers
Ability: Levitate
Tera Type: Steel
EVs: 244 Hp / 252 Def / 12 Spe
Bold Nature
IVs: 0 Atk
- Volt Switch
- Hydro Pump
- Will-O-Wisp
- Protect
Rotom is the defensive pivot of the team. Physically defensive Rotom-W can check many physical attackers with its bulk and Will-O-Wisp. Steel Tera gives Rotom-W a good defensive typing, which helps it check mons it would normally not check. Volt Switch generates momentum for the team and creates a VoltTurn core with Dragapult. Hydro Pump allows it to dent Ground Types that may try to deny Volt Switch. Will-O-Wisp can cripple physical attackers and provide chip damage. Protect allows Rotom-W to scout, get an extra turn of leftovers recovery, and potentially stall out burn damage.
Great Tusk @ Heavy-Duty Boots
Ability: Protosynthesis
Tera Type: Water
EVs: 252 HP / 252 Def / 4 SpD
Impish Nature
- Earthquake
- Rapid Spin
- Knock Off
- Stealth Rock
Great Tusk can take most physical attacks well, provide support, and dish out good damage when needed to. I gave it Heavy-Duty Boots so it wouldn't take hazard damage before it can remove them. Water Tera gives Great Tusk a good defensive typing. Earthquake gives Great Tusk a powerful STAB move which allows it to beat non-Air balloon Gholdengo, which may try to spinblock it. Rapid Spin removes hazards, and can also be used in a pinch to give Great Tusk a speed boost. Knock Off allows Great Tusk to remove held items, which goes well with hazards. Stealth Rock provides valuable chip damage to support wallbreakers.
Heatran @ Leftovers
Ability: Flash Fire
Tera Type: Grass
EVs: 252 HP / 216 SpD / 40 Spe
Calm Nature
IVs: 0 Atk
- Magma Storm
- Flash Cannon
- Tera Blast
- Taunt
Heatran can take special attacks, beat fairies, and still hit hard. Magma Storm allows Heatran to do big damage to a lot of mons. Flash Cannon allows Heatran to beat fairy types, which my team is weak to. Tera Blast helps Heatran beat Ground and Water types in a pinch after Terastalizing. Taunt gives Heatran a good supporting move, which helps against status users.
Iron Valiant @ Booster Energy
Ability: Quark Drive
Tera Type: Fairy
EVs: 4 Atk / 252 SpA / 252 Spe
Naive Nature
- Moonblast
- Close Combat
- Thunderbolt
- Psyshock
Iron Valiant is a reliable wallbreaker that hits hard from both the physical and special side. Speed Booster allows it to outspeed and cleanup late-game, or in a pinch, wallbreak earlier. Moonblast gives it a powerful and reliable Fairy move, nothing much to say. Close Combat gives Iron Valiant one of the most spammable moves in the game. Thunderbolt and Psyshock are good coverage moves on Iron Valiant. I've thought of replacing Iron Valiant with Contrary Enamorus-I, but I am not sure about it.
Gholdengo @ Covert Cloak
Ability: Good as Gold
Tera Type: Fighting
EVs: 252 SpA / 4 SpD / 252 Spe
Timid Nature
IVs: 0 Atk
- Make It Rain
- Shadow Ball
- Nasty Plot
- Recover
Gholdengo is another good wallbreaker that can spinblock and keep the Stealth Rock support that Iron Valiant needs. Covert Cloak is a good item in general, it also stops Gholdengo from taking damage from Salt Cure. Tera Fighting gives Gholdengo a defensive Tera Type that helps against Dark Type moves. Make It Rain gives Gholdengo a STAB 120 base power move. Shadow ball gives Gholdengo another good STAB move. Nasty Plot lets Gholdengo set up and increase its special attack stat by two stages. Recover gives Gholdengo health recovery, which gives it extra longevity.
I've used this team and had moderate success in the 1100 Elo range (I only recently started using showdown). I don't know if this team is good or not, even after testing, mostly because I don't know teambuilding very well, since I usually just use rental teams.
The Team
Dragapult @ Life OrbAbility: Infiltrator
Tera Type: Ghost
EVs 4 Atk / 252 SpA / 16 Spe
Hasty Nature
- Dragon Darts
- Shadow Ball
- Flamethrower
- U-turn
Dragapult works as the offensive pivot of the team. I chose Life Orb as the item because it provides extra power, allowing it to deal big damage. I used it over Specs/Band because I like the flexibility Life Orb provides on an offensive pivot. Dragon Darts is a good dragon move that can threaten dragons and nwutral mons, when weakened enough. Shadow Ball is Dragapult's most spammable move, and Ghost Tera gives it even more power. Flamethrower gives good coverage against Grass Types, which Rotom-W struggles with. U-turn generates momentum for the team and creates a VoltTurn core with Rotom-W.
Rotom-Wash @ Leftovers
Ability: Levitate
Tera Type: Steel
EVs: 244 Hp / 252 Def / 12 Spe
Bold Nature
IVs: 0 Atk
- Volt Switch
- Hydro Pump
- Will-O-Wisp
- Protect
Rotom is the defensive pivot of the team. Physically defensive Rotom-W can check many physical attackers with its bulk and Will-O-Wisp. Steel Tera gives Rotom-W a good defensive typing, which helps it check mons it would normally not check. Volt Switch generates momentum for the team and creates a VoltTurn core with Dragapult. Hydro Pump allows it to dent Ground Types that may try to deny Volt Switch. Will-O-Wisp can cripple physical attackers and provide chip damage. Protect allows Rotom-W to scout, get an extra turn of leftovers recovery, and potentially stall out burn damage.
Great Tusk @ Heavy-Duty Boots
Ability: Protosynthesis
Tera Type: Water
EVs: 252 HP / 252 Def / 4 SpD
Impish Nature
- Earthquake
- Rapid Spin
- Knock Off
- Stealth Rock
Great Tusk can take most physical attacks well, provide support, and dish out good damage when needed to. I gave it Heavy-Duty Boots so it wouldn't take hazard damage before it can remove them. Water Tera gives Great Tusk a good defensive typing. Earthquake gives Great Tusk a powerful STAB move which allows it to beat non-Air balloon Gholdengo, which may try to spinblock it. Rapid Spin removes hazards, and can also be used in a pinch to give Great Tusk a speed boost. Knock Off allows Great Tusk to remove held items, which goes well with hazards. Stealth Rock provides valuable chip damage to support wallbreakers.
Heatran @ Leftovers
Ability: Flash Fire
Tera Type: Grass
EVs: 252 HP / 216 SpD / 40 Spe
Calm Nature
IVs: 0 Atk
- Magma Storm
- Flash Cannon
- Tera Blast
- Taunt
Heatran can take special attacks, beat fairies, and still hit hard. Magma Storm allows Heatran to do big damage to a lot of mons. Flash Cannon allows Heatran to beat fairy types, which my team is weak to. Tera Blast helps Heatran beat Ground and Water types in a pinch after Terastalizing. Taunt gives Heatran a good supporting move, which helps against status users.
Iron Valiant @ Booster Energy
Ability: Quark Drive
Tera Type: Fairy
EVs: 4 Atk / 252 SpA / 252 Spe
Naive Nature
- Moonblast
- Close Combat
- Thunderbolt
- Psyshock
Iron Valiant is a reliable wallbreaker that hits hard from both the physical and special side. Speed Booster allows it to outspeed and cleanup late-game, or in a pinch, wallbreak earlier. Moonblast gives it a powerful and reliable Fairy move, nothing much to say. Close Combat gives Iron Valiant one of the most spammable moves in the game. Thunderbolt and Psyshock are good coverage moves on Iron Valiant. I've thought of replacing Iron Valiant with Contrary Enamorus-I, but I am not sure about it.
Gholdengo @ Covert Cloak
Ability: Good as Gold
Tera Type: Fighting
EVs: 252 SpA / 4 SpD / 252 Spe
Timid Nature
IVs: 0 Atk
- Make It Rain
- Shadow Ball
- Nasty Plot
- Recover
Gholdengo is another good wallbreaker that can spinblock and keep the Stealth Rock support that Iron Valiant needs. Covert Cloak is a good item in general, it also stops Gholdengo from taking damage from Salt Cure. Tera Fighting gives Gholdengo a defensive Tera Type that helps against Dark Type moves. Make It Rain gives Gholdengo a STAB 120 base power move. Shadow ball gives Gholdengo another good STAB move. Nasty Plot lets Gholdengo set up and increase its special attack stat by two stages. Recover gives Gholdengo health recovery, which gives it extra longevity.
I've used this team and had moderate success in the 1100 Elo range (I only recently started using showdown). I don't know if this team is good or not, even after testing, mostly because I don't know teambuilding very well, since I usually just use rental teams.
's set from whatever you are running now for a 
set instead. Dragapult is already pretty frail so chip damage (hazards, life orb and normal chip) are going to kill it eventually. With Specs, you have more power on the spa side and Draco Meteor will do much more damage. This is probably the most consistent set. You could also use a mixed set that is more focused on Status and being annoying with Dragapult's high speed.
for a more phys def set. With 
as your item, you want to be more bulky as that makes Gholdengo a reliable wall that won't have drops (defence drop from a crunch). I would change the tera from Fighting to Fairy. Now that Gholdengo is more defensive, we want a better defensive typing (fairy still resists dark moves).
for Protect. With taunt and magma-storm, you can stall out a pokemon while getting more health back from your leftovers. Protect is also good for scouting sets.
for 
on Great-Tusk. Rocky Helmet is better for punishing u-turns and also getting good chip on things that are helpfull for your other breakers like 
.
. Being able to hit ghosts for super effective damage is better coverage.
which is used for revenge killing.